Comparators for comparing two analog signals to one another and for indicating which of the two analog signals is the higher one are well known. One common circuit utilized to effect such comparison is the differential amplifier, which provides a positive output when one of the two signals is higher and a negative output when the other of the two signals is higher.
Typically, one of the inputs to the differential amplifier will be a reference voltage. The other input comprises a signal which is compared to the reference voltage, so as to obtain an indication as to which of the two inputs to the differential amplifier is higher.
Dedicated comparators, which are specifically designed to operate under open loop conditions and to function as switching devices, are also known. Such dedicated comparators function in a manner very similar to an open loop operational amplifier. One advantage of using such a dedicated comparator is that it frequently has a much faster response time than an operational amplifier which is used as a comparator.
Comparators have many different applications. They're commonly utilized in pulse generators, square-wave generators, triangular-wave generators, pulse-width modulators, level detectors, zero-crossing detectors, pulse generators, line receivers, limit comparators, voltage-controlled oscillators, A/D converters, and time delay generators, for example.
An important characteristic of a comparator is the response time or propagation delay introduced thereby. This is defined as the time between the input voltage transition and some specified point on the output voltage transition. Response times for contemporary voltage comparators generally range from about 1 .mu.s down to about 10 ns.
As those skilled in the art will appreciate, it is sometimes desirable to compare a signal to the higher of two different reference voltages. It is certainly possible to construct a three input comparator circuit using a plurality of contemporary comparators or the like. This may be accomplished simply by using a first comparator to compare the two reference voltages to determine which is the higher one thereof, then switching the highest reference voltage to the second comparator to compare the highest reference voltage to the signal. Alternatively, a signal can be compared to one reference voltage on a first comparator while simultaneously comparing the same signal to a second reference voltage on a second comparator, then applying logic gates to determine whether or not the signal is greater than both reference voltages.
However, to do so with contemporary devices inherently results in at least a doubling of the propagation delay. In some applications, such a propagation delay is not desirable.
In view of the foregoing, it is desirable to provide a three input comparator for comparing a signal to the greater of two reference voltages, wherein the three input comparator mitigates propagation delay and thus is suitable for use in applications requiring a fast response time.